A moving caris a complex symphony of sound, most of it reassuring—tires humming, engine rumbling, tailpipes roaring. But every now and then a dissonant note creeps into the mix—and the alert driver's ears prick up: Did you hear that? Here's a guide that identifies those oddball sounds and helps you sort out which conditions you can let slide and which ones mean repairs—and how fast you need to make them.

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WHERE: From one or more tires.

WHEN: At low speeds, especially in the morning. Speeds up with car.

WHAT: Flat-spotted tire. As the tire rotates the flat spot thumps on the ground. Nylon-cord tires will flat-spot overnight and make this sound until they warm up. It's usually worse in colder weather. You can permanently flat-spot the tires by locking up the wheels, grinding massive amounts of rubber off the tread in one spot.

FIX: Replace your nylon tires with steel-belted tires. If you've ground down your tires like a pencil eraser, you can either live with the vibration until wear reduces the flat-spotting or buy new tires.